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Problem with offset at second amplifier in AC coupled opamp

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cyberblak

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Hi all,

I use two high-gain (~30dB each) opamps in cascade. The amps are AC-coupled by a capacitor that forms a high-pass filter with the input impedance of the following opamp (2.5uF with 100 ohms = 1kHz cut-off freq.).

I need the AC coupling in order to avoid the offset voltage error from the first amp in the second stage (it would saturate it).

However there's an offset that appears at the terminal of the second amp, even if it's AC-coupled... This offset saturates the second amp, of course...

Any idea on what could happen?

Thanks,

cyberblak
 

pspice ac coupled

Maybe you should try another, or different type, capacitor ..
Its leakage current might cause the opamp to saturate ..

Regards,
IanP
 

Re: AC coupled opamp

Humm...

Should have mentionned that I got those results in simulation (Spice).

Thanks
 

Re: AC coupled opamp

what kind of spice model did you use for the opamp? An accurate model also simulates input offset currents and if the inbalance between the input resistance is great a high dc gain opamp may saturate. Can you give us the schematic and the opamp type?
 

Re: AC coupled opamp

I used two AD603 in cascade...

So the circuit is :

buffer amp -> cap -> AD603 -> cap -> AD603 -> buffer amp

thanks

Added after 1 hours 47 minutes:

I joined that part of the schematic.
 

Re: AC coupled opamp

Sorry the AD603 is no normal opamp and should present no offset even at high gain, maybe the model is incorrect or something wrong with the spice schematic.
 

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    cyberblak

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